Documentation
Data and Control Functions for all Modbus Protocol Flavours |
This Modbus protocol library implements the most commonly used data functions as well as some control functions. The functions to perform PLC program download and other device specific functions are outside the scope of this library.
All Bit Access and 16 Bits Access Modbus Function Codes have been implemented. In addition the most frequently used Diagnostics Function Codes have been implemented. This rich function set enables a user to solve nearly every Modbus data transfer problem.
The following table lists the supported Modbus function codes:
Function Code | Current Terminology | Classic Terminology |
|---|---|---|
Bit Access | ||
1 | Read Coils | Read Coil Status |
2 | Read Discrete Inputs | Read Input Status |
5 | Write Single Coil | Force Single Coil |
15 (0F hex) | Write Multiple Coils | Force Multiple Coils |
16 Bits Access | ||
3 | Read Multiple Registers | Read Holding Registers |
4 | Read Input Registers | Read Input Registers |
6 | Write Single Register | Preset Single Register |
16 (10 Hex) | Write Multiple Registers | Preset Multiple Registers |
22 (16 hex) | Mask Write Register | Mask Write 4X Register |
23 (17 hex) | Read/Write Multiple Registers | Read/Write 4X Registers |
Record Access | ||
20 (14 hex) | Read File Record | Read General Reference |
21 (15 hex) | Write File Record | Write General Reference |
Diagnostics | ||
7 | Read Exception Status | Read Exception Status |
8 subcode 00 | Diagnostics - Return Query Data | Diagnostics - Return Query Data |
8 subcode 01 | Diagnostics - Restart Communications Option | Diagnostics - Restart Communications Option |
43 (2B hex) subcode 14 (0E hex) | Read Device Identification | |
Vendor Specific | ||
Advantech | Send/Receive ADAM 5000/6000 ASCII commands |
When passing register numbers and discrete numbers to FieldTalk library functions you have to use the the Modbus register and discrete numbering scheme. See Register and Discrete Numbering Scheme. (Internally the functions will deduct 1 from the start register value before transmitting the value to the slave device.)
Using multiple instances of a MbusMaster... class enables concurrent protocol transfer on different communication channels (e.g. multiple TCP/IP sessions in separate threads or multiple COM ports in separate threads).